Described herein are dihydropyrazine ligands that can be used, for example, to synthesize metal complexes such as, without limitation, ruthenium and cobalt complexes for use as ALD or CVD precursors. Also described herein are complexes comprising dihydropyrazine ligands and methods for making or using same.
The electronics industry continues to source volatile metal containing precursors for vapor deposition processes, including chemical vapor deposition (CVD) and atomic layer deposition (ALD), for fabricating conformal metal containing films on substrates, such as silicon, metal nitride, metal oxide and other metal-containing layers, using these metal-containing precursors. In these techniques, a vapor of a volatile metal complex is introduced into a process chamber where it contacts the surface of a silicon wafer whereupon a chemical reaction occurs that deposits a thin film of pure metal or a metal compound. CVD occurs if the precursor reacts at the wafer surface either thermally or with a reagent added simultaneously into the process chamber and the film growth occurs in a steady state deposition. CVD can be applied in a continuous or pulsed mode to achieve the desired film thickness. In ALD, the precursor is chemisorbed onto the wafer as a self-saturating monolayer, excess unreacted precursor is purged away with an inert gas such as argon, then excess reagent is added which reacts with the monolayer of chemisorbed precursor to form metal or a metal compound. Excess reagent is then purged away with inert gas. This cycle can then be repeated multiple times to build up the metal or metal compound to a desired thickness with atomic precision since the chemisorption of precursor and reagent are self-limiting. ALD provides the deposition of ultra-thin yet continuous metal containing films with precise control of film thickness, excellent uniformity of film thickness and outstandingly conformal film growth to evenly coat deeply etched and highly convoluted structures such as interconnect vias and trenches. Suitable metal precursors for ALD include those which are thermally stable to preclude any thermal decomposition occurring during the chemisorption stage yet are chemically reactive towards added reagent. Additionally, it is important that the metal precursors are monomeric for maximum volatility and clean evaporation leaving only a trace of involatile residue. It is also desirable that the precursors have high solubility in hydrocarbon solvents to form solutions which can be used for Direct Liquid Injection (DLI) to deliver precursor vapor to the CVD or ALD reactor. Hydrocarbon solvents such as cyclooctane and mesitylene are particularly attractive since they are relatively high boiling points liquids and can be readily dried to low moisture levels.
Ruthenium and cobalt are particularly attractive metals for CVD and ALD processes in the fabrication of semiconductor devices. The deposition of ultra-thin films of ruthenium can be used to create electrodes in DRAM capacitor cells or provide a copper adhesion promoting thin film grown onto copper diffusion barrier materials such as titanium nitride or tantalum nitride. Ultra-thin continuous ruthenium films can also be used as seed layers upon which copper metal can be directly electroplated. Similarly, thin cobalt layers can also be applied as adhesion promoting films for titanium nitride or tantalum nitride. Alternatively, cobalt can be deposited as a ‘capping film’ onto copper interconnect lines. When depositing either metal onto titanium nitride, tantalum nitride or other substrates which can be reactive towards the element oxygen, it is especially desirable that the ruthenium and cobalt complexes do not contain the element oxygen as this will tend to form metal oxides which can lead to electrical failures within the device being fabricated.
There are numerous ruthenium precursors reported in the chemical literature, but a common process challenge encountered when using them in ALD is their long incubation times towards forming a continuous metal film and the need to use oxygen or ozone as a reagent. Long incubation times are a result of low metal atom deposition (nucleation) densities in the first ALD cycles which slowly increase with further cycles since the nuclei tend to act as sites for further metal deposition. With a sufficient nucleation density established a linear relationship between ALD film thickness and number of ALD cycles is established. In this way, as many as 500 initial ALD cycles can be required to establish a steady growth rate of a ruthenium film (S. Yim et al, Journal of Applied Physics, 103, 113509, 2008). Nucleation densities can be enhanced by the application of plasma during the ALD process, but the strong directional vectoring of plasma tends to degrade the uniformity of deposition compared to thermal ALD, especially on the vertical side walls of deeply etched structures which can be ‘shadowed’ from the plasma. On the other hand, the use of oxygen and ozone reagents can be a problematic in their ability to oxidatively damage barrier films such as titanium nitride and tantalum nitride and also lead to roughening and etching of the ruthenium metal. In this regard, there is a strong need to develop ruthenium precursors which can deposit ruthenium metal by chemically reductive processes to therefore avoid oxidative damage. Suitable reagents for reduction include, but are not limited to: hydrogen, ammonia, amines, hydrazines, silanes, alanes and boranes. Most highly desired processes would include combinations of ruthenium precursors with reducing agents where neither contains the element oxygen. Similarly, there is a need for reductive growth of cobalt metal films from oxygen free cobalt precursors under reducing conditions. So, in summary there is a need for oxygen free ruthenium and cobalt precursors which can deposit metallic ruthenium and cobalt by reduction.
Other metal precursors described in the prior art include, without limitation, one or more of the following cyclopentadienyl (Cp), pyrrole, imidazole, diene, CO, alkyl substituted phenyl, amidinates, guanidinates or combinations thereof. However, the ligands and complexes described herein differ from those in the prior art because they are based on non-aromatic dihydropyrazine ligand(s) which allow for high reactivity for growing metal films by ALD and CVD and are oxygen free.